Typically, a strong corona discharge is carried out on a laminate film to roughen the film face or to form micropores thereon prior to superimposing the laminate film on a subject sheet, in order to prevent generation of air pockets between the film and the subject sheet. In addition, during production of the laminate film, a woven fabric is layered on a thermoplastic synthetic resin film, followed by passing them through two hot rollers coming in contact with each other. Such a two-layer film is cooled, and the woven fabric layer is peeled off from the resin film, whereby embossed shapes are formed on the film to prevent the formation of air pockets.
However, since the laminate film face which is subjected to corona discharge or embossing treatment comprises discontinuously formed irregularities, passages capable of continuously discharging air captured between the film and the subject cannot be formed. Thus, formation of air pockets between the film and the subject after lamination treatment cannot be effectively prevented.
As another laminate film having further improved prevention effect of air pockets than the above mentioned film, there is developed a laminate film comprising very fine grooves continuously formed only in a longitudinal direction thereof. But, this film is also disadvantageous in that air pockets cannot be sufficiently prevented in the case where the subject sheet or the laminate sheet is applied with improper tension or width of the grooves is excessively broad.